The present invention relates to a signal concentration type data transmission system or the like.
In a terminal start type data transmission system, just as on the signal receiving side, the signal transmitting side must have the capacity for enabling a retransmission request when an error is detected in a received signal control code such as a response code (ENQ: enquiry) for a start request, a text acknowledgement code (ACK: acknowledgement), or a text negative acknowledgement code (NAK: negative acknowledgement). For reasons which will become apparent, in order to efficiently transmit a text, it is necessary to count how many times the retransmission request has been made.
A data transmission system of the type to which the invention relates is shown in block diagram form in FIG. 1. In this system, a data memory section 1 stores variable data and fixed data (hereinafter referred to as a "text" when applicable) to be transmitted to a terminal on the signal receiving side, namely, a master station such as a CPU. The data memory section 1 is connected to a transmitter 2. The transmitter 2 operates to convert parallel data outputted by the data memory section 1 into serial data and to transmit the text over a data channel.
The system further includes a receiver 3 which converts serial data received over the data channel into parallel data so that it can be read by the terminal and which also detects parity errors in various control codes. The output of the receiver 3 is connected to a comparison circuit section 4. The comparison circuit section 4 performs comparing operations on codes (control characters) received from the CPU via the receiver 3 with control characters from the data memory section 1, and applies an operation state signal representing the results of the comparison to a control section 5.
The control section 5 instructs the operations of the circuit elements and monitors the conditions of them, and controls a timer 6 and an error counter 7. The timer 6 is used for monitoring the time of data transmission between the CPU and the terminal on the signal transmission side, and the error counter 7 operates to count errors detected at various time instants during data transmission. The timer 6 and the error counter 7 supply state signals representing their states to the control section 5.
Data transmission from the described terminal is carried out as follows. When the signal transmitting side has data to be transmitted to the CPU, the data memory section 1 assembles a text, including a parity bit, and passes it to the transmitter 2 from which it is transmitted over the data channel. At the time of transmission, the timer 6 is set so that the receiver is made ready for receiving the data. If the data is not received by the receiver within the period of time set by the timer 6, then a stepping operation is performed by the error counter 7. Specifically, the content of the error counter 7 is increased by one and the text is transmitted again. When the content of the error counter 7 exceeds a predetermined value (usually three), it is determined that the data cannot be transmitted.
FIG. 2A shows what is present on the data channel in a conventional data transmission system. When the terminal on the text transmitting side is to transmit data, a start request ENQ is sent to the CPU to determine whether the data can be received or not. When the CPU is ready for receiving the data, the signal receiving side sends a response code ENQ to the signal transmitting side to initiate transmission of the text. As a result, the terminal on the signal transmitting side performs the transmission of the text. When the signal receiving side has correctly received the text, this fact is communicated to the signal transmitting side by sending a text acknowledgement code ACK. Thus, the signal transmitting side confirms that the data transmitting operation has been correctly carried out and the data transmission accomplished in the period of time set by the timer.
In the case where, as shown in FIG. 2B, it is detected that a response code from the signal receiving side or a transmitted text includes an error (the signal receiving side sending a text negative acknowledgement code NAK), the signal transmitting side transmits the start ENQ again or transmits the text again. The numbers of these operations are counted by the error counter 7. In the case when, although the content of the counter has reached the predetermined value, no response code ENQ or text acknowledgement code ACK is received, transmission is ended. In the case where, although the signal transmitting side has transmitted a start request ENQ, no response is made to the signal transmitting side (as shown in the FIG. 2C), the start request is transmitted at intervals corresponding to the period of time set by the timer 6.
The transmission of the start request can be carried out three times. If the number of times of transmission of the start request exceeds three, then it is determined that an error is involved, and the transmission procedure is commenced again. When the text acknowledgement code ACK is transmitted to the signal transmitting side, the comparison circuit section 4 detects whether or not it is an effective character, and the code is returned as an acknowledgement code ACK to the signal receiving side, as indicated by the pattern of FIG. 2A.
In the conventional data transmission system described above, if data transmission is carried out as indicated by the pattern of FIG. 2A, there is no problem. However, in the case where, as shown in FIG. 2B, a start request or text from the signal transmitting side contains an error and the negative acknowledgement code NAK is used for retransmission, the signal transmitting side performs similarly to the terminal on the signal receiving side, responding a predetermined number of times.
Accordingly, a signal concentration type data transmission system in which a single CPU is employed as a terminal on the signal receiving side and a plurality of terminal units on the signal transmitting side are connected to the CPU, the signal transmitting side requires an error counter and complex circuitry used in association with the error counter. In addition, during the operation of the counter 7, the response of ACK/NAK is continuously carried out. This is not efficient, increasing the overall transmission time. Furthermore, since the signal transmitting side transmits signals continuously, the CPU may be placed in a communication "panic" state.
An object of the invention is thus to eliminate the above-described drawbacks accompanying a conventional data transmission system. More specifically, an object of the invention is to provide a data transmission method in which a terminal on the signal transmitting side has a simple construction and with which transmission can be controlled mainly by a central terminal on the signal receiving side, and whereby, even in view of the simplified transmitting terminal construction, the transmission efficiency is improved and data is transmitted and received effectively and economically.